Treatment of diseases and ailments of the body often benefit from short- or long-term infusion of drugs and/or other fluids. While such therapeutic substances may be administered extracorporeally, e.g., via transcutaneous injection, many patients benefit from the consistent and repeatable dosage provided by an implantable infusion pump. Such pumps may be used in a variety of applications such as control of pain and/or spasticity. They are well-suited to deliver infusate fluids to a targeted delivery site such as an epidural or intrathecal space of the spinal canal, or a particular location within the brain.
Implantable infusion pumps are typically implanted subcutaneously, e.g., in the chest or abdominal cavity. The pump may incorporate a reservoir to hold the infusate fluid. A self-sealing, needle-penetrable septum may also be provided and is preferably located generally directly beneath the skin. The septum provides a fluid passageway that permits the reservoir to be refilled periodically via a transcutaneous injection. Accordingly, the pump reservoir can be filled or refilled without requiring surgical removal from the patient's body, and further without requiring any other significant surgical procedure.
The pump may also include a discharge outlet through which the therapeutic substance is directed during delivery. The outlet is typically connected to flexible medical tubing, e.g., a catheter, leading to the targeted delivery site. In addition to the reservoir, infusion pumps may further include a power source, a pump, and associated electronics to control delivery of the therapeutic substance to the patient in accordance with a prescribed schedule.
In many implantable infusion devices, a convoluted metal bellows serves as a reservoir for a therapeutic substance. The reservoir is contained within an outer housing that contains a propellant that acts on the bellows to maintain the therapeutic substance within the reservoir at a relatively constant pressure. In some devices that pressure is negative, i.e., below ambient pressure. One example of such a device is the MIP product marketed by Medtronic-Minimed. In other devices, the pressure at which the therapeutic substance is held is positive, i.e., above ambient pressure. One example of such a device is the SYNCHROMED device marketed by Medtronic, Inc.
It has been proposed that regardless of whether the infusion devices use negative or positive pressure reservoirs, differences between the pressure in the reservoir and the ambient pressure may vary undesirably. Attempts to address this issue have included the use of flexible reservoirs that are exposed to ambient pressure. One such approach is described in International Publication No. WO 03/099351 in which a reservoir at ambient pressure is used. Such reservoirs are sometimes referred to as “neutral pressure” reservoirs.
That approach may, however, suffer from its own disadvantages. For example, the reservoir uses a flexible bladder that is designed to store the therapeutic substance to the pump at ambient pressure. At ambient pressure, the flexible membranes may allow gases within the body to pass into the therapeutic substance, potentially creating problems for the operation of the pumps used to deliver the therapeutic substance to the patient. Another potential issue is in overfill protection. The devices may require a housing within which the flexible membrane is contained to prevent over-filling—thus adding additional complexity and bulk to the device.
Yet another potential disadvantage that may be associated with neutral pressure reservoirs is that gas bubbles may form within the therapeutic substances introduced into such reservoirs. The gas bubbles are typically formed from dissolved or entrained gas in the therapeutic substance. Bubble formation may be exacerbated by the increase in temperature typically associated with implanted reservoirs held at body temperature. Gas bubbles can potentially create problems in the pump mechanisms and/or in the catheter. To address this issue, the therapeutic substance may preferably be degassed before being loaded into the reservoir, although this adds additional complexity to the loading process.
Still other issues may be raised in connection with infusion devices that use peristaltic pumps to deliver a therapeutic substance from a reservoir to a catheter. Neutral pressure reservoirs may not provide sufficient fluid to adequately feed the inlet to a peristaltic pump. In some cases, the peristaltic pump tube may collapse at the pump inlet due to insufficient pressure to maintain the pump tube filled with the therapeutic substance. In devices that include a reservoir pressurized above ambient pressure (using, e.g., a propellant), the reservoir pressure may be undesirably increased due to temperature increases, changes in altitude, etc.
As the reservoir pressure increases, the occluding force applied to the pump tube may need to be increased to address reservoir-pressure induced leakage through the occluded portion of the pump tube. Frequently, the peristaltic pumps in such infusion devices are assembled in a significantly over-occluded state to ensure sufficient occlusion forces at all points on the pump tube. On the other hand, over-occlusion (excessive compressive load on the tube) is undesirable because it results in unnecessary friction and leads to increased wear and excessive power consumption by the pump drive system. Excessive power consumption is particularly undesirable in the field of implantable, battery-powered infusion devices. Over-occlusion results in increased cyclic loading of the pump tube and thus reduces the tube leakage safety margin and useful life. Similarly, over-occlusion results in unnecessary wear on the pump rollers, bearings and other components.